The present invention relates to high speed driving method and mechanism of a pressure cylinder which is suitably utilized when a work is pressurized at an end of a stroke like a gun cylinder for spot welding.
A movable-side electrode driven by a gun cylinder for spot welding is opposed to a work on a stationary-side electrode through a relatively short working stroke, and it is required that spot welding is carried out plural times in minimal time while deviating welding positions.
To satisfy such a requirement, in Japanese Patent Application No.2001-49020 proposed by the present inventors, a mechanism for setting an intermediate stopping position of a piston is provided so that the piston can be stopped at an intermediate position. A totally returning position to which a work moves to the opposed position of a piston rod and an operation preparing position (intermediate stopping position) to which the piston rod is allowed to be opposed to the work through a relatively short working stroke are set in the piston, and when the spot welding is carried out plural times, the working stroke is made as short as possible, and efficiency of operation is enhanced.
However, the intermediate stopping position is stationary, and is always fixed irrespective of a shape of the work and various conditions. That is, as a stopping position when the piston rod is retreated, there are only two positions, i.e., the operation preparing position (intermediate stopping position) and the totally returning position, and even if the work has an uneven portion, the intermediate stopping position can not be changed in accordance with the uneven portion, and it is always necessary to select one of the two positions. As a result, wasteful stroke is generated in accordance with a work shape in some cases and in such a case, there is a problem that the operation time unnecessarily becomes long.
Further, in a conventional pressure cylinder driving mechanism including the above proposed art, if the cylinder is driven by a switching valve, it is necessary to fill a pressure cylinder whose pressure has completely been lowered to atmospheric pressure with compressed air when the cylinder starts its forward or backward stroke. Therefore, it takes time to supply pressure, and the driving operation of the pressure cylinder is delayed correspondingly, and this largely affects the working efficiency.
The above-described problem is seen not only in the spot welding gun cylinder, but also in a device in which a head mounted to a tip end of a piston rod is repeatedly pushed against a work such as a pressure unit of various clamp device, and such a case also has the same problem.
To solve the technical problem, the present invention provides a driving method and a driving apparatus in which a totally returning position and an operation preparing position (intermediate stopping position) to which a piston rod is opposed to a work through a relatively short working stroke are set in the piston rod of a pressure cylinder such as a spot welding gun, the intermediate stopping position can arbitrarily be set by a signal sent from outside so that a waste is not generated in a stroke in accordance with a shape of the work and the like.
To achieve the above object, according to the present invention, there is provided a high speed driving method of a pressure cylinder comprising: an advancing working step for driving a piston of a pressure cylinder by air pressure which is supplied from and discharged from a servo valve to move a piston rod to a forward stroke end, thereby pressurizing the work to carry out a predetermined operation; and an intermediate stopping step for retreating the piston rod to an intermediate stopping position and stopping the piston rod at this position.
The intermediate stopping step comprises: a process for detecting an operation position of the piston rod by a position detector and feeding back a detected position signal to a control apparatus of the servo valve; a process for controlling the servo valve by the compressed air such that a deviation between the detected position signal and a set position signal for intermediate stopping the piston rod becomes zero, thereby intermediate stopping the piston rod at a set position; and a process for maintaining a state in which substantially the same pressures are supplied to pressure chambers on opposite sides of the piston.
In the driving method of the invention, it is preferable that when the advancing working step is started, a given voltage Vmax is supplied to the servo valve to allow the servo valve to function as on/off switching valve. With this, the piston rod can move forward at high speed.
In the driving method of the invention, it is preferable that a speed in the vicinity of an end of the forward stroke of the piston rod is reduced by cushion effect caused by limiting a flow rate of exhaust gas from a rod-side pressure chamber. With this, a tip end of the piston rod can touch a work softly.
When the cushion effect functions, an internal pressure in the rod-side pressure chamber becomes lower than an internal pressure in ahead-side pressure chamber, this acts as a trigger to actuate a quick exhaust valve, the quick exhaust valve directly opens the rod-side pressure chamber into atmosphere to abruptly lower a back pressure of the piston, so that time required until a maximum pressurizing force can be generated by the piston rod is shortened.
According to the present invention, to carry out the driving method, there is provided a high speed driving apparatus of a pressure cylinder, comprising: a pressure cylinder having a piston driven by an air pressure, a piston rod connected to the piston, a head-side pressure chamber and a rod-side pressure chamber formed on opposite sides of the piston; a servo valve connecting both the pressure chambers to an air pressure source; a control section for controlling the servo valve; a position detector for detecting an operation position of the piston rod to feed back a detected position signal to the control section.
The high speed driving apparatus further comprises a control apparatus which functions such that the control section compares the detected position signal fed back from the position detector and a set position signal for intermediate stopping the piston rod as previously set, and controls the servo valve such that a deviation between both the signals becomes zero, thereby intermediate stopping the piston rod at a set position, and a state in which substantially the same pressures are supplied to pressure chambers formed on opposite sides of the piston is maintained.
In the invention, it is preferable that the control section further comprises a signal circuit which supplies a given voltage Vmax to the servo valve when the advancing working step of the piston rod is started to allow the servo valve to function as an on/off switching valve, and a switch which is actuated by the switch signal to selectively connect the signal circuit and the control apparatus to the servo valve.
The pressure cylinder may comprise a cushion mechanism for reducing a speed of the piston rod in the vicinity of the end of the forward stroke of the piston rod, thereby limiting a flow rate of exhaust gas from the rod-side pressure chamber.
Further, it is possible to employ a structure in which a quick exhaust valve is connected to the pressure cylinder, if an internal pressure in the rod-side pressure chamber at the time of the cushion effect functions becomes lower than an internal pressure in the head-side pressure chamber, this acts as a trigger to actuate the quick exhaust valve to directly open the rod-side pressure chamber into atmosphere.
According to the invention having the above structure, the intermediate stopping position of the piston rod can arbitrarily be set by a signal input to the control apparatus from outside. Therefore, if an intermediate stopping position is previously obtained in accordance with a shape of a work and the intermediate stopping position is input as a designated position successively, it is possible to prevent a waste from being generated in stroke.
Further, as described above, since the state in which substantially the same pressures are applied to the pressure chambers formed on the opposite sides of the piston is maintained when the piston rod is returned to the intermediate stopping position, when a projecting stroke is started, it is unnecessary to fill the pressure chamber whose pressure is completely lowered to the atmospheric pressure with compressed air, and it is only necessary to add pressure fluid of necessary amount for previously added pressure fluid. Therefore, there is no driving delay unlike the conventional case in which a pressure chamber is driven by a switching valve, time required for projecting stroke can be shortened and thus, operation efficiency can be enhanced.